As electronics evolve, there is an increased need for miniature switches that are provided on semiconductor substrates along with other semiconductor components to form various types of circuits. These miniature switches often act as relays, and are generally referred to as micro-electro-mechanical system (MEMS) switches. In many applications, MEMS switches may replace field effect transistors (FETs), and are configured as switches to reduce insertion losses due to added resistance as well as parasitic capacitance and inductance inherent in providing FET switches in a signal path. MEMS switches are currently being considered in many radio frequency (RF) applications, such as antenna switches, load switches, transmit/receive switches, tuning switches, and the like.
Turning to FIGS. 1A and 1B, a MEMS device 10 having a MEMS switch 12 is illustrated according to one embodiment of the present invention. The MEMS switch 12 is formed on an appropriate substrate 14. The MEMS switch 12 includes a movable member, such as a cantilever 16, which is formed from a conductive material, such as gold. The cantilever 16 has a first end and a second end. The first end is coupled to the substrate 14 by an anchor 18. The first end of the cantilever 16 is also electrically coupled to a first conductive pad 20 at or near the point where the cantilever 16 is anchored to the semiconductor substrate 14. Notably, the first conductive pad 20 may play a role in anchoring the first end of the cantilever 16 to the semiconductor substrate 14 as depicted.
The second end of the cantilever 16 forms or is provided with a cantilever contact 22, which is suspended over a contact portion 24 of a second conductive pad 26. Thus, when the MEMS switch 12 is actuated, the cantilever 16 moves the cantilever contact 22 into electrical contact with the contact portion 24 of the second conductive pad 26 to electrically connect the first conductive pad 20 to the second conductive pad 26. The MEMS switch 12 may be encapsulated by one or more encapsulating layers 30, which form a substantially hermetically sealed cavity about the cantilever 16. The cavity is generally filled with an inert gas and sealed in a near vacuum state. Once the encapsulation layers 30 are in place, an overmold material 32 may be provided over the encapsulation layers 30 as part of a high volume packaging process.
To actuate the MEMS switch 12, and in particular to cause the cantilever 16 to move the cantilever contact 22 into contact with the contact portion 24 of the second conductive pad 26, an actuator plate 28 is disposed over a portion of the substrate 14 and under the middle portion of the cantilever 16. To actuate the MEMS switch 12, an electrostatic voltage is applied to the actuator plate 28. The presence of the electrostatic voltage over time creates a field that moves the metallic cantilever 16 toward the actuator plate 28, thus moving the cantilever 16 from the position illustrated in FIG. 1A to the position illustrated in FIG. 1B.
Unfortunately, actuation of a MEMS switch 12, especially one maintained at near vacuum conditions, results in the cantilever 16 moving downward with a momentum sufficient to cause the cantilever contact 22 to bounce one or more times off of the contact portion 24 of the second conductive pad 26 after initial contact. Such bouncing degrades circuit performance and effectively increases the closing time. The article entitled “A Dynamic Model, Including Contact Bounce, of an Electrostatically Actuated Microswitch,” by Brian McCarthy et al., provides a detailed analysis of this bouncing phenomenon and is incorporated herein by reference. The dynamic closing forces may also be sufficient to damage both the contact portion 24 of the second conductive pad 26 as well as the cantilever contact 22, thus causing excessive wear, which results in a shortened operating life for the MEMS switch 12.
As a result, efforts have been made to control the force at which the cantilever 16 is pulled down to reduce bouncing. In particular, an actuation signal having a special waveform is initially applied to the actuator plate 28. The actuation signal moves the cantilever 16 downward, such that the contact pad 22 at the end of the cantilever 16 initially moves rapidly toward the contact portion 24 of the second conductive pad 26. The actuation signal is configured such that the effective electrostatic voltage is reduced or removed prior to the cantilever contact 22 coming into contact with the contact portion 24 of the second conductive pad 26. The downward momentum will continue to move the cantilever 16 downward, albeit at a decreasing rate, wherein the contact pad 22 lands softly and slowly on the contact portion 24 of the second conductive pad 26. Once the MEMS switch 12 is closed, a hold signal is applied to actuator plate 28 to hold the cantilever 16 in a closed position such that the contact pad 22 is held in contact with the contact portion 24 of the second conductive pad 26. The article “A Soft-Landing Waveform for Actuation of a Single-Pole Single-Throw Ohmic RF MEMS Switch,” by David A. Czaplewski et al., provides a technique for providing a pre-determined actuation signal to control the closing of a MEMS switch 12 and is incorporated herein by reference.
Providing an actuation signal to effect soft closings of the MEMS switches 12 theoretically reduces bouncing and increases the operating life of the device. In practice however, process variation in the switch manufacture will reduce or eliminate the efficiency of a single waveform to effect soft closing as described.
For example, if the gap between the cantilever 16 and the actuator plate 28 increases due to manufacturing variation, a nominal actuation signal may not be strong enough to move the cantilever 16 enough to provide a soft closing. As such, when the hold signal is subsequently applied, bouncing may occur if the cantilever contact 22 is not proximate the contact portion 24 of the second conductive pad 26. Conversely, if the gap between the cantilever 16 and the actuator plate 28 decreases due to manufacturing variation the nominal actuation signal may be too much, thus causing a hard closing, which may induce bouncing or damage. Further, humidity, temperature, aging, and wear may play a role in changing the mechanical characteristics, and thus operation, of MEMS switches 12. Accordingly, there is a need for a technique to reduce or eliminate bouncing in MEMS switches 12 over various process variations and operating conditions.
MEMS switches 12 also have issues associated with being released from a closed position, or opening. The cantilever 16 is effectively a metallic beam, which is deflected when the MEMS switch 12 is closed and suspended in a natural state when the MEMS switch 12 is open. Releasing the MEMS switch 16 entails turning off the hold signal, and thus releasing the deflected cantilever 16 from the closed position. Once released, the cantilever 16 springs upward and begins mechanically oscillating up and down. Such mechanical oscillation is referred to as ringing, and in a cavity in a near vacuum state this ringing may continue for an extended period of time. Further, the magnitude and time of ringing may vary over various operating conditions and process variations.
If the cantilever 16 is still ringing when the next actuation signal is applied, the nominal actuation signal may not provide a soft closing given the cantilever's position, upward momentum, downward momentum, or a combination thereof. And, during this ringing, the electrical isolation provided by the switch may be reduced, effectively prolonging the true opening time of the switch. Accordingly, there is a further need for a technique to reduce or eliminate ringing of MEMS switches 12 over various operating conditions and process variations.